On Jan 19, 2018, at 7:07 AM, Jim Wiggs <[email protected]> wrote: > > I've been told by quite a few folks that splitting the SSD between log and > cache is "a bad idea" or "suboptimal" but frankly, I don't buy it.
There are two reasons for this. 1. The illumos ZFS driver will enable the on disk native write cache only if it's given a whole disk (i.e., the whole_disk property is set true). This is due in part to legacy reasons involving UFS. If the whole_disk property is not set then the write cache will not be modified, which may mean that it's WCD. 2. Even if your disks are WCE or your partitions all have whole_disk set true, if your partitions are not properly aligned you can end up with read-modify-write overhead. Either of these cases may be sufficient to kill performance. And that's not to say that you ought not use partitions, or that performance will always be bad. If you know what you're doing, and if handled properly there won't be any performance penalty for using slices. I wouldn't do it managing thousands of pools in a datacenter, but if it works for you and it saves you some money, rock on. See the OpenZFS wiki for some more detail: http://www.open-zfs.org/wiki/Performance_tuning#Whole_Disks_versus_Partitions <http://www.open-zfs.org/wiki/Performance_tuning#Whole_Disks_versus_Partitions> -- Brian Bennett Systems Engineer, Cloud Operations Joyent, Inc. | www.joyent.com <http://www.joyent.com/>
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